GHC.Types.Hint

An "unknown" hint. This type constructor allows arbitrary -- hints to be embedded. The typical use case would be GHC plugins -- willing to emit hints alongside their custom diagnostics.

Suggests adding a particular language extension. GHC will do its best trying to guess when the user is using the syntax of a particular language extension without having the relevant extension enabled.

Example: If the user uses the keyword "mdo" (and we are in a monadic block), but the relevant extension is not enabled, GHC will emit a 'SuggestExtension RecursiveDo'.

Test case(s): parsershould_failT12429, parsershould_failT8501c, parsershould_failT18251e, ... (and many more)

Suggests possible corrections of a misspelled pragma. Its argument represents all applicable suggestions.

Example: {-# LNGUAGE BangPatterns #-}

Test case(s): parsershould_compileT21589

Suggests that a monadic code block is probably missing a "do" keyword.

Example: main = putStrLn "hello" putStrLn "world"

Test case(s): parsershould_failT8501a, parsershould_failreadFail007, parsershould_failInfixAppPatErr, parsershould_failT984

Suggests that a "let" expression is needed in a "do" block.

Test cases: None (that explicitly test this particular hint is emitted).

Suggests to add an ".hsig" signature file to the Cabal manifest.

Triggered by: DriverUnexpectedSignature, if Cabal is being used.

Example: See comment of DriverUnexpectedSignature.

Test case(s): driver/T12955

Suggests to explicitly list the instantiations for the signatures in the GHC invocation command.

Triggered by: DriverUnexpectedSignature, if Cabal is not being used.

Example: See comment of DriverUnexpectedSignature.

Test case(s): driver/T12955

Suggests to use spaces instead of tabs.

Triggered by: PsWarnTab.

Examples: None Test Case(s): None

Suggests adding a whitespace after the given symbol.

Examples: None Test Case(s): parsershould_compileT18834a.hs

Suggests adding a whitespace around the given operator symbol, as it might be repurposed as special syntax by a future language extension. The second parameter is how such operator occurred, if in a prefix, suffix or tight infix position.

Triggered by: PsWarnOperatorWhitespace.

Example: h a b = a+b -- not OK, no spaces around +.

Test Case(s): parsershould_compileT18834b.hs

Suggests wrapping an expression in parentheses

Examples: None Test Case(s): None

Suggests to increase the -fmax-pmcheck-models limit for the pattern match checker.

Triggered by: DsMaxPmCheckModelsReached

Test case(s): pmcheckshould_compileTooManyDeltas pmcheckshould_compileTooManyDeltas pmcheckshould_compileT11822

Suggests adding a type signature, typically to resolve ambiguity or help GHC inferring types.

Suggests to explicitly discard the result of a monadic action by binding the result to the _ wilcard.

Example: main = do _ <- getCurrentTime

Suggests adding an identifier to the export list of a signature.

Suggests increasing the limit for the number of iterations in the simplifier.

Suggests to explicitly import Type from the Data.Kind module, because using "*" to mean Type relies on the StarIsType extension, which will become deprecated in the future.

Triggered by: PsWarnStarIsType Example: None Test case(s): wcompat-warnings/WCompatWarningsOn.hs

Suggests placing the qualified keyword after the module name.

Triggered by: PsWarnImportPreQualified Example: None Test case(s): module/mod184.hs

Suggests using TemplateHaskell quotation syntax.

Triggered by: PsErrEmptyDoubleQuotes only if TemplateHaskell is enabled. Example: None Test case(s): parsershould_failT13450TH.hs

Suggests alternative roles in case we found an illegal one.

Triggered by: PsErrIllegalRoleName Example: None Test case(s): rolesshould_failRoles7.hs

Suggests qualifying the * operator in modules where StarIsType is enabled.

Triggered by: PsWarnStarBinder Test case(s): warningsshould_compileStarBinder.hs

Suggests that for a type signature 'M.x :: ...' the qualifier should be omitted in order to be accepted by GHC.

Triggered by: PsErrInvalidTypeSignature Test case(s): module/mod98

Suggests to move an orphan instance (for a typeclass or a type or data family), or to newtype-wrap it.

Triggered by: TcRnOrphanInstance Test cases(s): warningsshould_compileT9178 typecheckshould_compileT4912 indexed-typesshould_compileT22717_fam_orph

Suggests to use a standalone deriving declaration when GHC can't derive a typeclass instance in a trivial way.

Triggered by: GHC.Tc.Errors.Types.DerivBadErrConstructor Test cases(s): typecheckshould_failtcfail086

Suggests to add a standalone kind signature when GHC can't perform kind inference.

Triggered by: TcRnInvisBndrWithoutSig Test case(s): typecheckshould_failT22560_fail_d

Suggests the user to fill in the wildcard constraint to disambiguate which constraint that is.

Example: deriving instance _ => Eq (Foo f a)

Triggered by: GHC.Tc.Errors.Types.DerivBadErrConstructor Test cases(s): partial-sigsshould_failT13324_fail2

Suggests to use the appropriate Template Haskell tick: a single tick for a term-level NameSpace, or a double tick for a type-level NameSpace.

Triggered by: TcRnIncorrectNameSpace.

Suggests enabling -ddump-splices to help debug an issue when a Name is not in scope or is used in multiple different namespaces (e.g. both as a data constructor and a type constructor).

Concomitant with NoExactName or SameName errors, see e.g. "GHC.Rename.Env.lookupExactOcc_either". Test cases: T5971, T7241, T13937.

Suggests adding a tick to refer to something which has been promoted to the type level, e.g. a data constructor.

Test cases: T9778, T19984.

Something is split off from its corresponding declaration. For example, a datatype is given a role declaration in a different module.

Test cases: T495, T8485, T2713, T5533.

Suggest a similar name that the user might have meant, e.g. suggest traverse when the user has written travrese.

Test case: mod73.

Remind the user that the field selector has been suppressed because of -XNoFieldSelectors.

Test cases: NFSSuppressed, records-nofieldselectors.

Suggest importing from a module, removing a hiding clause, or explain to the user that we couldn't find a module with the given ModuleName.

Test cases: mod28, mod36, mod87, mod114, ...

Found a pragma in the body of a module, suggest placing it in the header.

Suggest using pattern matching syntax for a non-bidirectional pattern synonym

Test cases: patsynshould_failrecord-exquant typecheckshould_failT3176

Suggest tips for making a definition visible for the purpose of writing a SPECIALISE pragma for it in a different module.

Test cases: none

Suggest renaming implicitly quantified type variable in case it captures a term's name.

Suggest enabling one of the SafeHaskell modes Safe, Unsafe or Trustworthy.

Suggest removing a record wildcard from a pattern when it doesn't bind anything useful.

Suggest moving a method implementation to a different instance to its superclass that defines the canonical version of the method.

Suggest to increase the solver maximum reduction depth

Suggest removing a method implementation when a superclass defines the canonical version of that method.

Suggest eta-reducing a type synonym used in the implementation of abstract data.

Remind the user that there is no field of a type and name in the record, constructors are in the usual order $x$, $r$, $a$

Suggest binding the type variable on the LHS of the type declaration

Suggest using an anonymous wildcard instead of a named wildcard

Suggest explicitly quantifying a type variable instead of relying on implicit quantification

Suggest binding explicitly; e.g data T @k (a :: F k) = ....

An unknown binding (i.e. too complicated to turn into a Name)

Suggest to enable the input extension. This is the hint that GHC emits if this is not a "known" fix, i.e. this is GHC giving its best guess on what extension might be necessary to make a certain program compile. For example, GHC might suggests to enable BlockArguments when the user simply formatted incorrectly the input program, so GHC here is trying to be as helpful as possible. If the input SDoc is not empty, it will contain some extra information about the why the extension is required, but it's totally irrelevant/redundant for IDEs and other tools.

Suggest to enable the input extensions. The list is to be intended as disjunctive i.e. the user is suggested to enable any of the extensions listed. If the input SDoc is not empty, it will contain some extra information about the why the extensions are required, but it's totally irrelevant/redundant for IDEs and other tools.

Suggest to enable the input extensions. The list is to be intended as conjunctive i.e. the user is suggested to enable all the extensions listed. If the input SDoc is not empty, it will contain some extra information about the why the extensions are required, but it's totally irrelevant/redundant for IDEs and other tools.

Suggest to enable the input extension in order to fix a certain problem. This is the suggestion that GHC emits when is more-or-less clear "what's going on". For example, if both DeriveAnyClass and GeneralizedNewtypeDeriving are turned on, the right thing to do is to enabled DerivingStrategies, so in contrast to SuggestSingleExtension GHC will be a bit more "imperative" (i.e. "Use X Y Z in order to ... "). If the input SDoc is not empty, it will contain some extra information about the why the extensions are required, but it's totally irrelevant/redundant for IDEs and other tools.

Some module exports what we want, but we aren't explicitly importing it.

Some module exports what we want, but we are explicitly hiding it.

The module exports what we want, but it isn't a type.

The module exports what we want, but it's a type and we have ExplicitNamespaces on.

Suggest importing a data constructor to bring it into scope

It was locally bound at this particular source location.

It was imported by this particular import declaration.